I hate to fall in here.... I must have misunderstood.... I thought fets were voltage driven not current... I have always assumed only the gate capacitance needs attention!! Why would you need a current driver?
It's not my area of expertise so I genuinely want to know..
Hi Ian,
You have not misunderstood. FETs, including MOSFETs, are with no doubt voltage driven.
The gate/source voltage defines the drain current when the MOSFET drain is saturated. Saturated is where the drain/source voltage is above a level when an increase of drain/source voltage does not cause the drain current to increase. Vacuum tubes are the same. Below the drain/source saturation voltage MOSFETs behave like resistors, where a decrease in drain/source voltage causes a decrease in drain current and vice versa.
In pulse width modulation applications, MOSFETs are either turned hard on or completely off. When MOSFETs are turned hard on they behaves like a resistor. When turned hard on, high power MOSFETs may have drain/source resistances as low as 10 mili Ohms, while small signal MOSFETs may have drain/source resistances as high as 100 Ohms.
And that would be that if it were not for two things, speed of switching the MOSFET on and off and the big influencing factor, gate effective capacitance which is huge on power MOSFETs.
A high-power MOSFETs may have actual gate/source capacitances of 20nF (not an error). But that is not the whole story. There is another big capacitor from the drain to the gate (same as the Miller capacitor in vacuum tubes). The rub with the drain/gate capacitor is that it forms a virtual capacitance at the gate. And worse still, the capacitance reflected to the gate is multiplied by the voltage gain of the MOSFET, (gm * Z drain). On fast switching MOSFETs, typical of PWM circuits, Z drain is high and thus the voltage gain is high and thus the capacitance reflected to the gate is correspondingly high, say a further 20nF. On MOSFET data sheets, the parameter that gives an indication of the effective gate source capacitance is gate charge.
So after all that, the high power MOSFET has an effective input capacitance of 40nF.
Finally to the crux of the matter. To make the gate/source voltage change fast you need to provide a high source and sink current to charge and discharge the effective gate/source capacitance fast. For example, some MOSFET gate drivers have a seven-amp source and sink current capability, but the average current capability is around 1.5A.
There are other reasons why a high current is required to drive MOSFET gates, but charging and discharging the effective gate/source capacitance is the main one.
And if you do not turn MOSFETs on and off fast, there is a danger that the MOSFET's safe operating area will be exceeded and the MOSFET will be destroyed. Also, a lot of power will be wasted because the MOSFET will be in the linear region for a long time. You can also get oscillations in some pulse width modulation circuits with slow gate voltages.
I hope I have not bored you too much.
spec